A general physical mechanism of earthquake preparation and occurrence discussed theoretically in the paper is that an elastic“pull-loosened uplift—compressional depression”unstable structure is formed and developed gradually in vertical direction in a seismogenic zone before earthquake and two elastic compressional zones and two elastic pull-loosened zones symmetrically and alternatively distributed can be also formed in horizontal direction. This bi-directional strain structure in which the strain“compressional and pull-loosened zones are formed synchronously and exist oppositely and together form an entirety”is an essentially difference for formation of seismogenic body and non-seismogenic body produced by the crustal tectonic movement and also the main basis for finding the seismogenic zone. The
mechanism for seismic fracturing is that the bi-directional strain structure formed in a seismogenic zone is the necessary condition for instability and shear fracturing after stress concentration. An initial fracture after its formation can cause the compressional zones and pull-loosened zones to reverse expand and contract and in a coordinated pattern to uninterruptedly provide a dislocation space, and hence to make the stress and fracture to rapidly transmit and extend. This knowledge of the mechanisms for earthquake preparation and fracturing can be used to explain most of the seismic precursory phenomena and sand blow, water ejection, and fault rupture and dislocation.